Effects of Tip Stiffness on Piles with down Drag

The loading response of a single vertical pile was calibrated against historical data on strain gauge loads acquired at a highway bridge abutment on Steele County Route 7 in Owatonna, Minnesota. At this site, H-type piles were driven to a weathered bedrock layer, and soil surcharging was used to reduce the anticipated settlement of a layer of clayey sand overburden 15 m (50 ft) thick. Evaluating the load in the bridge piling for both this case study and subsequent sensitivity modeling provides comprehensive information to support the Minnesota Department of Transportation's revised drag-load design. The simulation was performed with FLAC3D software, and the goal of the model was to investigate behavior of piles subjected to negative skin friction (down drag). A sensitivity analysis was conducted from the calibrated case by changing the stiffness of the strata along the frictional portion of the pile and in the end-bearing strata. The simulations showed that, for a relative stiffness below 0.1 (very stiff base layer), the neutral plane was at the bottom of the pile, and maximum possible drag-load forces were realized. At a relative stiffness above 10 (very soft base layer), the neutral plane was near the top of the pile, and the drag-load force was minimal. This research suggests that, for many Minnesota state transportation projects, the drag load is centered between the extreme cases. Findings indicate that, for piles driven to stiff rock, drag load must be evaluated to ensure pile structural capacity is sufficient; historically, this check was often ignored.